Flow boiling is a common phenomenon in modern technology and industries. A finite difference Front Tracking method is used to study film boiling on a flat plate subject to a constant heat flux. Large density ratios are examined (1000, 2000). The flow structure including the circulation zones, the vortex developing areas, and vortex pair generation are studied in detail. The numerical method is validated by comparing the result with the analytic solution for a simple problem. Large bubbles comparable to the computational domain are formed at large density ratios. The bubble either breaks up and departs from the rest of the gas, or a jet is formed depending on the heat flux imposed. It is found that the Nusselt number depends on the heat flux on the wall. Specifically, the Nusselt number decreases as the heat flux is raised. The Morton number also affects the Nusselt number in agreement with experimental correlations. The Nusselt number increases when the Morton number is raised. The results obtained agree with experimental correlations at relatively low heat fluxes. Agreement gets worse at large heat fluxes. Experimental correlations estimate larger Nusselt numbers compared to the present study. The flow enhances along the direction of the gravitational acceleration at large density ratios (2000). A wide neck is formed that stabilizes at a specific position inside the flow. The microstructure of the flow shows a vortex pair that develops beneath the root of the bubble at a large density ratio (2000).